Frankly I have a similar issue.
Running Roundcube towards my webhost to check mail when I am not able to invoke my default local mailprogram.
I am a "user", thus I have zero access (that I know of) to my webhosts Roundcube configuration and this is
driving me nuts because they don't seem to be able to help me either (i will link this thread to them though).
My desktop and browser is in dark mode, all my desktops are in dark mode, I set the Roundcube Elastic Theme to Light because the Dark is just not very appealing to me, and it endures the session, but whenever getting to it on another browser, another computer or just restarting the browser I am back at Elastic Dark mode.
Please code a persistent setting or I will just have to go back to Squirrelmail or something else... how can you not have it?